1. Field of the Invention
The invention is related to the areas of buffers and buffer amplifiers, such as unity gain buffers and replica amplifiers.
2. Description of the Related Art
Buffer circuits are used to provide buffering between a first circuit and a second circuit. In other words, a buffer circuit is normally used to transfer voltage from a first circuit, having a high impedance level, to a second circuit with a lower impedance level. The interposed buffer circuit prevents the first circuit from unacceptably loading the second circuit and thus interfering with its desired operation.
If the voltage transferred is unchanged, the amplifier is known as a unity gain buffer. A simple unity gain buffer may be constructed by connecting the output of an operational amplifier to its inverting input, and using the non-inverting input. Other unity gain buffers may include an emitter follower, Darlington pair, or similar configurations using field effect transistors, vacuum tubes or other active devices.
Two preferred traits of a buffer amplifier are high input impedance and low output impedance. In addition, succeeding stages should be linear so as not to introduce distortion. As a result, there can be difficulty involved in designing a buffer stage that is linear and has sufficient bandwidth to process the signal with enough linearity such that distortions are not introduced. Feedback is typically used in this situation. However, there are limitations with respect to the amount of feedback that may be applied, the bandwidth of the feedback, and the loop gain that results.
In a related system, replica amplifiers may be used within a replica structure in order to predict a normal amplifier response and then couple the replica amplifier with a normal feedback amplifier such that the feedback system now only needs to do small corrective changes to the output. For example, if driving 1V into 1-ohm load, a replica amp may be used to supply the bulk of the current and the feedback amplifier can then operate with much lower currents.
Nevertheless, achieving both wide bandwidth and high linearity can be challenging. The previous topologies, discussed above, do not adequately address the need for wide bandwidth and high linearity within desirable power limitations. The claimed invention, however, is able to achieve higher linearity than previous solutions for a given power and area budget.